MGB - poor running...
Discussion
OK, here goes....
1979 MGB GT as standard as you get apart from breakerless ignition system. Running fairly well until recently but after a (shortish) period of non-use it's running like a dog. On investigation I checked the timing to find that, at tickover, on removing the vacuum advance pipe the timing moves about 30 degrees - is that normal? Never seen it on any other motor.
The poor running is missing badly and popping/banging when opening the throttle quickly from low rpm, difficulty starting and lack of power. I'm pretty sure it's an ignition issue but the vacuum advance timing change really surprised me.
Thoughts?
1979 MGB GT as standard as you get apart from breakerless ignition system. Running fairly well until recently but after a (shortish) period of non-use it's running like a dog. On investigation I checked the timing to find that, at tickover, on removing the vacuum advance pipe the timing moves about 30 degrees - is that normal? Never seen it on any other motor.
The poor running is missing badly and popping/banging when opening the throttle quickly from low rpm, difficulty starting and lack of power. I'm pretty sure it's an ignition issue but the vacuum advance timing change really surprised me.
Thoughts?
Initial thought is we need more info...
How long is "shortish"?
Have you tried new petrol?
30deg seems a bit high but could be close dependent on the Vac model fitted. Some vac units go as high as 24deg. Check out Paul Hunt's web site for more info...
http://www.mgb-stuff.org.uk/ignitiontext.htm
Best of....
MGmike
How long is "shortish"?
Have you tried new petrol?
30deg seems a bit high but could be close dependent on the Vac model fitted. Some vac units go as high as 24deg. Check out Paul Hunt's web site for more info...
http://www.mgb-stuff.org.uk/ignitiontext.htm
Best of....
MGmike
Shortish = 3 weeks. It was being used 50 miles a day 3 days a week but now down to 20 miles a week or so.
Timing could be moving 24 deg, it just looked a hell of a lot for it to move at 700rpm. Wouldn't have thought there was that much depression but I guess it depends on the springs fitted to the vac advance.
Timing could be moving 24 deg, it just looked a hell of a lot for it to move at 700rpm. Wouldn't have thought there was that much depression but I guess it depends on the springs fitted to the vac advance.
Three weeks is not shortish, three months is shortish 
The manifold depression is at it's greatest on overrun and only slightly lower at idle. There's loads of good sites which explain how to use a vac gauge as a troubleshooting tool. Check them out.
Back to the problem....
It probably is ignition related but before you go chucking parts at it you need to do a few standard checks (Nigel will be proud of me for this
)
Check and clean the plugs, leads, dizzy cap, rotor arm and coil HT output for any damage/tracking. Check the pipe from the Servo to the inlet manifold is not split and, as a test to confirm the servo isn't faulty, remove the pipe and block the connection on the inlet manifold.
If none of that works, come back for more suggestions.
Best of.....
MGmike
The manifold depression is at it's greatest on overrun and only slightly lower at idle. There's loads of good sites which explain how to use a vac gauge as a troubleshooting tool. Check them out.
Back to the problem....
It probably is ignition related but before you go chucking parts at it you need to do a few standard checks (Nigel will be proud of me for this
)Check and clean the plugs, leads, dizzy cap, rotor arm and coil HT output for any damage/tracking. Check the pipe from the Servo to the inlet manifold is not split and, as a test to confirm the servo isn't faulty, remove the pipe and block the connection on the inlet manifold.
If none of that works, come back for more suggestions.
Best of.....
MGmike
if it's had any new p*ss poorly made new parts fitted in recent times then they can play up and be difficult to pin down when problems occur
if you've got a list of new ignition and fuel parts or components fitted fairly recently that might help to track things down or at least check they've been installed correctly
favourites are CB points, condenser, rotor arms and even plugs and dissy caps
also there are poor installation of igniter heads, the delicate wires could be scuffed by the installation not allowing for movement of the plate, many times when these are rejected because of problems it's the installation at fault not the unit but many don't like to admit it
to set the car up it needs to be fully serviced then it's this order tappets, CB points, plugs, timing, carb mixture
if you adjust any item in that chain then you'll need to check/adjust the items that follow it in the chain
three weeks non-use shouldn't make any difference but long periods of non-use does the car no favours
if you've got a list of new ignition and fuel parts or components fitted fairly recently that might help to track things down or at least check they've been installed correctly
favourites are CB points, condenser, rotor arms and even plugs and dissy caps
also there are poor installation of igniter heads, the delicate wires could be scuffed by the installation not allowing for movement of the plate, many times when these are rejected because of problems it's the installation at fault not the unit but many don't like to admit it
to set the car up it needs to be fully serviced then it's this order tappets, CB points, plugs, timing, carb mixture
if you adjust any item in that chain then you'll need to check/adjust the items that follow it in the chain
three weeks non-use shouldn't make any difference but long periods of non-use does the car no favours
Edited by nta16 on Thursday 19th December 15:30
I agree with the need to get decent quality parts. The car was properly set up about 2000 miles ago by someone who I do trust and it went well until, as I say, about a 3 week lay off. I think it got damp and that certainly didn't help. Perhaps it's just damp still and I'm not driving it sufficiently to get it all dry. It is garaged but still get cold / damp.
Perhaps new leads / dizzy cap etc. will help but it's the big change in timing when I disconnect the vac advance that has surprised me. Usually, one disconnects this when setting the timing but I am reticent to do this as it has been set up and the dizzy not moved since but it is running as if the timing is way out.
Phil.
Perhaps new leads / dizzy cap etc. will help but it's the big change in timing when I disconnect the vac advance that has surprised me. Usually, one disconnects this when setting the timing but I am reticent to do this as it has been set up and the dizzy not moved since but it is running as if the timing is way out.
Phil.
Phil, forget about how much it moves for now as the dizzy "could" be introducing that amount.
What's the measured timing setting at tickover with the advance disconnected? With this as the start point we can establish a known base to work from. Don't adjust it, just record it as a reference point for now.
If the car was set-up only 2k back and ran well, all the settings should be okay. That indicates you need to look for a defective component somewhere but as I said earlier, don't chuck parts at it until you work out what's going on.
For a garaged car I can't see damp as the main cause but we'll see as you progress. Clean the cap and leads with a clean dry cloth and check the points gap is still as it should be.
Best of...
MGmike
What's the measured timing setting at tickover with the advance disconnected? With this as the start point we can establish a known base to work from. Don't adjust it, just record it as a reference point for now.
If the car was set-up only 2k back and ran well, all the settings should be okay. That indicates you need to look for a defective component somewhere but as I said earlier, don't chuck parts at it until you work out what's going on.
For a garaged car I can't see damp as the main cause but we'll see as you progress. Clean the cap and leads with a clean dry cloth and check the points gap is still as it should be.
Best of...
MGmike
depends what you mean by you got it damp, do you mean wet - either way as said make sure all ignition electrics are clean and dry, as well as above clean and dry the inside of the dissy cap
any more info you can give, new parts fitted, what has changed/happened since it was running well and it not
ETA: MGmike, bear in mind some garages are damp too
any more info you can give, new parts fitted, what has changed/happened since it was running well and it not
ETA: MGmike, bear in mind some garages are damp too
Edited by nta16 on Friday 20th December 13:21
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